The present invention relates in general to a method and an apparatus for melting metal ingots, so-called "pig", and more particularly to such method and apparatus wherein a charge material, e.g., a metal ingot pile consisting of a plurality of horizontal arrays of metal ingots superposed on each other, is introduced into a vertically extending melting chamber, and melted therein.
In the art of melting apparatus having a vertically extending melting chamber for melting metal ingots, there has been known a vertical melting furnace which is usually referred to as a "tower melter". As disclosed in journals "Al-Aru", pages 27-32 (March, 1982), and "MODERN METALS", Vol. 38, No. 11, P76 (1982), such melting apparatus commonly includes a vertically extending columnar melting furnace having a top lid closing its upper open end. Metal ingot piles or blocks each consisting of plural metal ingots superposed on each other in a stack are introduced through the upper open end of the melting furnace, by use of suitable loading or charging equipment. The introduced piles of metal ingots are pre-heated while they are gradually lowered in to the furnace, and are melted under heat by burners provided in the lower part of the furnace. The thus obtained molten metal is reserved in a reservoir which communicates with the lower part of the furnace.
In such a vertical melting furnace, the piles of metal ingots introduced in the furnace through its top opening are moved downward as the previously introduced lower piles are melted by the burners. While the metal ingot piles are lowered, they are pre-heated by exhaust gases of the burners flowing upward through the charged furnace. Consequently, the piles of metal ingots which have descended down to the lower part of the furnace become more or less molten, and may be readily melted by the burners.
In this type of melting apparatus wherein the metal ingot piles are directly dropped into a vertical furnace through its upper open end, however, the introduced metal ingot piles may frequently be caught or suspended halfway or part way through the furnace. This phenomenon may easily happen, particularly because the metal ingots are piled in such manner that the pile is difficult to collapse sideways or in lateral directions. If the metal ingot piles remain halfway through the falling distance in the furnace, the flames of the burners will not reach the metal ingots of the suspended piles. Further, these ingots are not sufficiently heated by the exhaust gases of the burners. In this case, therefore, extra heating time is required for melting those metal ingots, and the melting efficiency or economy is reduced.
Moreover, the above-described manner of charging the vertical melting furnace with metal ingot piles will not permit the metal ingots in each pile to be sufficiently separated from each other and evenly distributed within the furnace. In other words, the metal ingots dropped into the furnace tend to remain in a piled or stacked condition. As a result, there exist spaces or voids between the inner wall surface of the furnace and the metal ingot piles. In this state, the exhaust gases blow upward through such voids. This may reduce the efficiency of heat transfer from the exhaust gases to the metal ingots, and cause local melting of the ingots in the pre-heating portion of the furnace, resulting in increased possibility of local blow of the exhaust gases. The exhaust gas blow and consequent localization of the exhaust gas flow has an adverse effect on the pre-heating efficiency of the metal ingots during their downward movement. Further, the melting of the metal ingots in the pre-heating zone of the furnace which is distant from the lower reservoir, will cause oxidization of the molten metal, with an unfavourable result of increasing an amount of dross in the melt.
Another type of melting apparatus for piles of metal ingots is known as a high-speed melting furnace in which the piles of metal ingots are moved in succession through a horizontally extending channel, and melted by burners at the end of the channel. The molten metal is dripped down into a lower dry hearth, through an opening which is formed through the bottom wall of the melting chamber. The molten metal poured in the dry hearth is then led into a reservoir. In this type of melting furnace, the metal melt is dripped a relatively long distance from the upper melting chamber down to the lower dry hearth, and is therefore subject to considerable thermal loss as well as unit loss due to oxidization. Consequently, it is necessary to heat the molten metal in the dry hearth before it is led into the reservoir. Thus, the known high-speed melting furnace is disadvantageous in terms of heating efficiency and cost of heating equipment.
For improved melting efficiency in the above-described type of high-speed melting furnace, it is essential that the metal ingot material moved to the melting end of the melting chamber be melted while both solid and liquid pahses coexist at equilibrium, before the melt is poured down through the opening in the melting portion. However, it is very difficult to maintain such conditions, in view of the possibility of changes in the charge material and configuration of the charge material (metal ingots). In the case of aluminum ingots, the range of temperature at which solid and liquid phases may coexist is relatively narrow. Namely, the aluminum ingot is melted progressively from its outer portion. Hence, the application of the conventional high-speed melting furnace to aluminum ingots is extremely difficult.